Combustion engines are machines that convert chemical energy stored in fuel into mechanical energy useful for generating electricity, producing thrust, or otherwise doing work. These engines typically include several cooperative sections that contribute in some way to the energy conversion process. In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustion section. The products of combustion are harnessed and directed through a turbine section, where they expand and turn a central rotor shaft. The rotor shaft may, in turn, be linked to devices such as an electric generator to produce electricity.
To increase efficiency, engines are typically operated near the limits of the engine components. For example, to maximize the amount of energy available for conversion into electricity, the products of combustion (also referred to as the working gas or working fluid) often exit the combustion section at high temperature and velocity. This elevated temperature and velocity generates a large amount of potential energy, but also places a great deal of stress on the downstream components, such as the blades and vanes of the turbine section.
The above-mentioned turbine section typically includes matched blades and vanes which are grouped together into coordinating sets known as “stages”. These blades and vanes have airfoil-shaped body regions and include end shrouds that help fix them in place inside an engine. Several, typically four, axially-spaced stages of matched blades and vanes cooperatively interact with the hot working fluid, which is forced at high speed through the turbine section, to spin the rotor shaft. Over time, exposure to the working gas elevated temperature and velocity may lead to component failure.
The first two stages of blades and vanes in an industrial gas turbine engine are exposed to a stream of working fluid that is extremely hot (above 2000° F.) and moving very quickly (above 500 ft/s). The blades and vanes in this environment must tolerate not only extreme thermal loads, but high-magnitude dynamic loads, as well. As a result, these components are traditionally rugged, internally-cooled structures that often include external thermal barrier coatings.
Unfortunately, while robust architecture and barrier coatings help the blades and vanes withstand external thermal and mechanical loads, they do not address all of the issues associated with exposure to the working fluid. For example, non-uniform temperature distribution between the cooled airfoil portions and relatively hot shroud portions introduces thermal gradients that produce internal thermal stresses. Cooling channel exits also produce localized thermal stresses, by inducing thermal gradients in the areas immediately surrounding the exits, as a result of sharp drops in temperature.
These internal stresses act in addition to the existing external thermal and mechanical loads to produce an elevated amount of composite stress within the blades and vanes. That is, external thermal stress due to the extreme heat of the working fluid, mechanical stresses due to the extreme velocity of the working fluid, and internal thermal stress due to thermal gradients within the vane segment all contribute the overall, composite stress level in this region. If this cumulative or “composite” stress at a given point exceeds a threshold amount, component failure may be accelerated or spontaneously induced. Therefore, even if external thermal and mechanical stress levels are kept below corresponding individual limits, the aggregate impact of these stresses, along with thermal-gradient-induced internal stresses, at a given point may be high enough produce to component failure. Accordingly, although modern blades and vanes are typically able to withstand external thermal and mechanical loads, additional internal loads induced by thermal gradients may, as a group, produce “composite” stress levels that exceed a failure-inducing threshold value.
One particular problem is the amount of composite stress concentrated in vane segments at the interface between the shroud portions and airfoil-shaped body portions due to typically-high levels of stress from three distinct sources that act in a cumulative manner in this region: external thermal stresses and mechanical stresses (induced by direct interaction with the working fluid), and internal thermal stresses (induced by thermal gradients within the vane).
Although thermal gradients may be present to a certain degree in many engine components, they are especially prevalent in the vanes of the turbine second stage or row. Row two vanes are often mounted in a cantilevered fashion to permit free rotation of the engine rotor shaft, with an outer end attached to the turbine casing and an inner end that is left free. This makes these vanes prone to cracking issues, because they must, without being supported at both ends, still withstand the extreme mechanical and thermal loads induced by the working fluid. As a result, of this requirement, row two vane segments are often especially robust. For example, to provide the stiffness required to withstand forces transmitted by the working fluid, row two vane segments may span two or more airfoils and typically include end shrouds that are particularly thick. Unfortunately, while this type of arrangement helps the vane segments withstand external loads, the increased rigidity (actually)/tends to make these components more susceptible to thermal gradients and the stresses associated therewith. Various approaches have been taken to reduce the presence of thermal gradients in second row vanes, with each approach achieving varying degrees of success.
The external thermal and mechanical loads introduced on engine components result largely from the of operating conditions required to meet power output demands. These components of composite stress are, accordingly, not easily reduced without negatively impacting engine performance. As a result, lowering thermal and mechanical loads is not a viable approach to reducing composite stress levels. Accordingly, addressing the internal thermal gradient stress represents the only practical means for reducing overall, composite stress levels.
Thermal-gradient-induced stresses come largely as a result of interaction between hot regions and cool regions within a given component, as engine the component seeks to reach thermal equilibrium. Therefore, there are three main factors which influence the impact of thermal gradients: the amount of heat which must be transferred and the area available for transfer of heat and the thickness/length of the feature.
As noted above, the amount of heat which needs to be transferred between regions of a component is one of three key factors that impact the affect of thermal gradients. One way to reduce this amount is simply to reduce the overall operating temperature of the engine. A second way is to increase the amount of internal cooling that is used. While each of these approaches may be used to partially reduce the overall amount of heat transferred within second row turbine vanes, both methods have drawbacks. For example, lowering the overall operating temperature reduces the temperature within the vanes, but reduces the energy available for conversion into electricity. Similarly, although increasing the use of internal cooling may lower the amount of heat remaining to be dissipated within a given vane, this approach may also dramatically reduce operating efficiency. Furthermore, increasing the amount of internal cooling may also create additional internal stresses by introducing additional localized thermal loads due to local thermal gradients surrounding the cooling locations. For a variety of reasons, reducing the amount heat to be transferred is often not a feasible approach to reducing thermal gradient stress.
As also noted above, the rate at which heat is transferred between component regions is a second factor that affects the impact of thermal gradients: excessive rates of heat transfer lead to cracking, while more moderate rates of transfer allow for extended part life. One way to reduce heat transfer rate is to increase the area of contact between hot and cold regions, such as by providing filleted joints where regions of disparate temperature meet. Fillets are used because, in addition to providing extra contact useful for heat transfer, their geometry reinforces the vane against the mechanical stresses that tend to induce cracks along right-angled or other non-curved joints.
The use of fillets to increase mass as a way to lower heat transfer rates has a practical limit in this environment, however. A fillet having enough mass to effectively reduce heat transfer rates becomes so large that it may actually store heat, rather than dissipate it. A further drawback to the “enlarged fillet” approach lies in the need to ensure proper airflow through the turbine section. Adding material to the vane segment may disrupt the aerodynamics of the vane profile, thereby disturbing airflow through the turbine and reducing the efficiency of the energy conversion process.
Therefore, there remains a need in this art for a crack resistant vane segment that maintains composite stress levels that are below an accepted threshold value. The vane segment should maintain this acceptable level of composite stress by reducing heat transfer rates without storing heat or negatively impacting vane aerodynamics. In addition, the vane segment should address thermal gradient issues without sacrificing performance. To this end, the vane segment should provide a cooling arrangement that lowers internal thermal stresses without requiring performance-inhibiting temperature reductions, without increasing the amount of cooling fluid used, and without introducing thermal gradients.